In a system for constraining a passenger in an automotive vehicle upon collision, Japanese Unexamined Patent Application Publication No. 10-217818 discloses a passenger constraining apparatus in which an inflatable bag is arranged between a seat cushion and a seat pan, so that a front portion of the seat cushion is pushed upward by inflating the bag upon collision of the vehicle to prevent a submarine phenomenon that the passenger tends to slip through a lower side of a lap belt upon front collision even though a seatbelt is fastened.